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Dim Headlight on a 1979 Yamaha XS750

by Keith Fellenstein

| 7/26/2011 9:42:52 AM

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Editor's note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with "Keith's Garage" as your subject.

Dim headlight on a 1979 Yamaha XS750Q: After a 25-year hiatus from riding, I decided to get back into motorcycling a little while back. I am really enjoying the magazine and your new column. The newer bikes don’t hold the same appeal that vintage Seventies and Eighties bikes do. I purchased a low-mileage 1979 Yamaha XS750, which I have restored and have been riding for the past year. As we are about to get back into prime riding season here in the North, I am continuing to struggle with a problem involving a dim headlight that I am hoping you might help with.

The problem involves a rather dim output. The headlight is a new halogen type, replaced last year. The light dims at idle, which is something that I would expect due to low output of the alternator at low engine speeds, and does get brighter when RPM rises about 1,500 or so. It still seems really dim, however. The interesting part of this is that when I apply either the front or rear brake enough that the brake light is activated, the brightness of the headlight increases about 30 percent, by my estimate — enough that the light becomes useful! I initially assumed a bad ground that was somehow improved as a result of the brake light circuit, but the schematics do not seem to support this, and I subsequently ran a separate ground wire direct from the headlight to the battery, with no change. As a side note, this same effect is not seen when using other electricals, such as turn signals.

I am now considering bypassing the reserve lighting unit to see if that could be causing the problem, but feel like I am chasing my tail. Your thoughts? - Chris Delling/Sterling Heights, Mich.

A: Thanks for the compliment. I’m having a blast writing this column, so it’s good to hear folks are enjoying it. You’re probably on the right track considering the reserve lighting unit. The reserve lighting unit on the Yamaha XS750 is one of those things engineers love but riders hate. It watches the headlight circuit and is supposed to sense a burned out filament in the headlight and switch to the other filament. If the high beam is defective, it switches on the low beam. If the low beam is defective, it switches to the high beam, but reduces the voltage to the light to dim it. Then it lights up a dashboard light to tell you your headlight is burned out. I don’t know about you, but I can usually tell if my headlight is burned out by the road in front of me being dark at night.

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Since your headlight is dim all the time, except when braking, I’d suspect the reserve lighting unit has gone bad. If you remove it, you have to jumper some wires to restore your lights to their normal function. The connections you need to make are blue/black to blue/yellow. That should give you normal headlight function and allow the high beam indicator to work. You can jump the circuit temporarily to test this, and if it fixes the problem, you can cut the connector off of the reserve lighting unit and connect the proper wires together. That way if you should ever want to replace the reserve lighting unit, you haven’t modified the wire loom. MC

For a 30+ y.o. bike, I'd make the recommendation to follow the 80/20 rule instead of suspecting the [transistorized, low failure rate] reserve headlight unit first, ie: 80% of problems on a motorcycle are electrical in nature, and 20% of those will be from a bad ground. The stock wiring to the headlight is usually too small, generating a lot of light loss right there [a 12v 55w halogen running at 11v only puts out something like 45w of light: a 20% drop for a 10% drop in voltage. I forget the exact numbers, but it's something that defies the laws of physics at first glance iirc.] A bad ground can completely overwhelm the f/x of the niggardly choice of wire gauge taken by the manufacturer's accountants: clean those contacts! Try running a separate ground wire from the headlight shell back to the upper motor mount, and pull apart all those silly bullet connectors that were used simply because they make assembly easier, not for reliability, and either polish & reassemble them with a little copper anti-seize [keeps the oxygen out & conducts the magic smoke surprisingly well] or cut them out & solder the wires back together properly: have you any idea how many Honda stators have be sacrificed to weathered Molex connectors? [Can't use the anti-seize on Molex connectors due to the potential for short-circuits; better to cut them out completely & solder, as was official Honda factory recall procedure for 1000s of GoldWings, VFRs & others...]

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